A vertical cavity surface emitting laser (VCSEL) is a semiconductor laser emitting light in a direction perpendicular to a substrate surface, and in comparison to an edge emitting type semiconductor laser, has features of low cost, low power consumption, small-sized, high-performance, and being able to be easily integrated two-dimensionally.
A vertical cavity surface emitting laser has a resonator structure including a resonator area that includes an active layer, and an upper reflecting mirror and a lower reflecting mirror provided above and below the resonator area, respectively (see Patent document No. 1, mentioned later). Accordingly, the resonator area is formed to have a predetermined optical thickness such that resonance of light of a wavelength λ occurs in the resonator area in order to obtain light of the oscillation wavelength λ. The upper and lower reflecting mirrors are formed by alternately laminating and forming materials having different refractive indexes, i.e., a low refractive index material(s) and a high refractive index material(s). In order to obtain high reflectance at the wavelength of λ, the upper and lower reflecting mirrors are formed in such a manner that the optical thickness of the low refractive index material(s) and the high refractive index material(s) is λ/4. It has also been proposed to form elements having different wavelengths in a chip (see Patent documents Nos. 2, 3, 4 and 7, mentioned later).
An atomic clock (atomic oscillator) is a clock that can measure time very precisely. The technology of miniaturizing and so forth of an atomic clock has been studied. An atomic clock is an oscillator that is based on transition energy amounts of electrons included in atoms such as an alkali metal. In particular, it is possible to obtain a very precise value of transition energy in electrons of atoms of an alkali metal in a state of having no disturbance. As a result, it is possible to obtain frequency stability higher by several digits than a crystal oscillator.
From among several types of atomic clocks, an atomic clock of a Coherent Population Trapping (CPT) system has frequency stability higher than a crystal oscillator by an order of about three digits in comparison to a crystal oscillator of the prior art, and it is possible to expect an extremely compact size and extremely low power consumption (see Non-patent documents Nos. 1 and 2, and Patent document No, 5, mentioned later).
An atomic clock of a CPT system has, as shown in FIG. 1, a light source 910 of a laser element or the like, an alkali metal cell 940 in which an alkali metal is sealed, and a light detector 950 that receives laser light that has been transmitted by the alkali metal cell 940. The laser light is modulated, and two transitions of electrons in alkali metal atoms are carried out simultaneously at side band wavelengths that appear on both sides of a carrier wave that is a specific wavelength, whereby the laser light is excited. The transition energy in the transitions does not change, and a transparency phenomenon of absorptivity of light being reduced in the alkali metal occurs when the side band wavelengths coincide with the wavelengths corresponding to the transitions. In the atomic clock, the wavelength of the carrier wave is adjusted so that the absorptivity of light of the alkali metal is thus reduced, and a signal detected in the light detector 950 is fed back to the modulator 960. The modulation frequency of the laser light that is emitted from the light source 910 such as a laser element is thus adjusted by the modulator 960. It is noted that the laser light is emitted from the light source 910, and is incident on the alkali metal cell 940 through a collimator lens 920 and a λ/4 plate 930.
In FIG. 1, MF denotes a magnetic field; L denotes the optical path length of laser light passing through the alkali metal; D denotes a diameter of the laser light; and X denotes a distance.
As a light source of such an extremely small-sized atomic clock, a vertical cavity surface emitting laser that is small-sized, having extremely low power consumption, and having high wavelength quality is suitable. As wavelength accuracy of a carrier wave, ±1 nm with respect to a specific wavelength is needed (see Patent document No. 3).
The frequency stability of an atomic clock is limited by either the shorter of the diameter D or the optical path length L of the laser light passing through the alkali metal cell 940. The shorter either the shorter of the diameter D or the optical path length L becomes, the worse the stability becomes. Accordingly, it is preferable that the diameter D of the laser light is as large as possible.
However, the divergence angle of laser light of a vertical cavity surface emitting laser is narrower than an edge emitting type laser. Accordingly, in a case where the frequency stability is to be increased, it is necessary to elongate the distance X between the light source 910 and the collimator lens 920 in order to increase the diameter D of the laser light passing through the alkali metal cell 940. Thus, in a case where a vertical cavity surface emitting laser is used as the light source 910, it may be difficult to satisfy both miniaturization of the atomic clock and high frequency stability.
Further, manufacturing a large number of vertical cavity surface emitting lasers that oscillate at the same wavelength may be difficult due to an influence of a variation in growth speed, a variation in distribution of a film thickness and so forth of a semiconductor layer during manufacture. Thus, vertical cavity surface emitting lasers may have a problem concerning reproducibility and uniformity in vertical cavity surface emitting lasers thus manufactured. Specifically, a film formed by a common Metal Organic Chemical Vapor Deposition (MOCVD) apparatus or a Molecular Beam Epitaxy (MBE) apparatus has film thickness uniformity on the order of 1% through 2%. As a result, in a case where a film having a thickness the same as a wavelength 850 nm is formed, an in-plane distribution of 8.5 nm through 17 nm may occur. Thus, for uses for which on the order of ±1 nm with respect to a wavelength is needed, yield may be reduced, and cost may be increased.